1. Field of the Invention
This invention relates to use of nuclear magnetic resonance (NMR) imaging techniques, and in particular, to use of multiple frequencies for determination of gas saturation radial profiles.
2. Description of the Related Art
Various instruments applying Nuclear Magnetic Resonance (NMR) imaging technology are useful for measuring certain petrophysical properties of earth formations. NMR well logging instruments typically include a magnet for polarizing nuclei in the earth formation in the vicinity of a wellbore. The polarizing typically occurs along a static magnetic field; at least one antenna is used for transmitting radio frequency (“RF”) energy pulses into the formations, which manipulates spins for desired measurements. The magnitude of the RF energy emitted by the precessing nuclei and the rate at which the magnitude changes are related to certain petrophysical properties of interest in the earth formations.
A typical embodiment of an NMR logging instrument for characterization of geologic deposits includes a side-looking or “centralized” NMR logging instrument. Typically, the instrument operates using a gradient magnetic field and multiple frequencies ƒ. One example of such an instrument is the MR ExplorerSM provided by Baker Hughes, Incorporated of Houston Tex. (referred to as the “MREX instrument,” the “logging instrument” or simply as the “instrument” herein).
There are several principal operating parameters in NMR well logging. These parameters should be optimized for efficient operation of an NMR well logging instrument. Such parameters include the logging speed (speed of motion of the instrument along the wellbore), the average and the peak power supplied to the instrument and transmitted as RF pulses, and the signal-to-noise ratio (“SNR”). Other parameters of interest include the vertical resolution of the instrument and the radial depth of investigation of the measurements made by the instrument within the formations surrounding the wellbore.
Physical parameters of particular interest to wellbore operators are the fractional volume of pore spaces in the earth formations (“porosity”), the texture of the rock and connectivity of the pore spaces, and the nature of the fluids contained in the pore spaces. Typical petroleum bearing earth formations contain water and hydrocarbon; some pores may be filled with water and others with hydrocarbons. Since hydrocarbons generally have different NMR relaxation properties than water, various NMR relaxometry techniques have been developed to qualitatively determine the nature of the fluids present in certain earth formations.
One method, for example, enables discriminating between gas and oil, and light oil and water. This method includes performing NMR spin-echo experiments using two different “wait times”, Tw. The wait time Tw is the delay between individual Carr-Purcell-Meiboom-Gill (“CPMG”) spin echo measurement sequences. See S. Meiboom et al, Rev. of Sci. Instr. v. 29, p. 6881 (1958). Another technique, described in U.S. Pat. No. 5,498,960 issued to Vinegar et al, uses two different inter-echo spacing times, Te, for CPMG sequences measured in a gradient magnetic field. Typically, the inter-echo spacing time Te is the time between rephasing radio frequency (RF) energy pulses applied to the logging instrument's antenna to “rephase” precessing nuclei which are influenced by the NMR survey. The rephasing RF pulses result in the “spin echoes” whose amplitude is measured. Gas, oil and water generally have different self-diffusivities, and these differences will be reflected in differences in the apparent transverse relaxation time T2 calculated for an earth formation between CPMG sequences measured using different values of the inter-echo spacing time Te. The technique described in the Vinegar et al. '960 patent for discriminating types of fluids in pore spaces of earth formations typically uses two values of the inter-echo spacing time Te.
In addition to the multiple inter-echo spacing time Te and multiple wait time Tw acquisitions, the use of multiple frequencies fin NMR measurements enhances aspects of formation evaluation. State of the art NMR logging instruments have a depth of investigation (DOI), (interchangeably referred to as a “radial depth”) less than about five (5) inches deep into a formation. Thus, the sensitive volume is typically flushed or invaded by mud filtrate. The difference in the depth of investigation associated with different frequencies makes it possible to study the variation of invasion within the span of NMR sensitive volumes. Such variation may be more observable for the gas reservoir, because the mobility of the gas is highest among all reservoir fluid types. By processing frequency data separately, it is possible to observe variation in gas saturations should it occur. However, since NMR sensitive volume span is limited to few inches only, the variation in the flushed zone saturation is limited, and the consistency of the results processed with individual-frequency data may be compromised for high-noise data.
Invasion can be seen as a process of replacement of movable formation fluids by mud filtrates introduced by drilling of a well. For a well having water-based drilling mud, the hydrocarbon saturation becomes smaller in the invaded zone due to the invasion of the water-based mud filtrate. For a well having oil-based drilling mud, the hydrocarbon saturation in the invaded zone could be increased from the native oil saturation (such as the case where there is movable water) or relatively unchanged. Gas saturation, Sg, is always reduced or intact in the invaded zone when using drilling mud that is either one of water-based or oil-based. In order to account for the varying possibilities, it is necessary to simultaneously use all frequency data in the processing.
Therefore, what are needed are techniques for processing of data for multiple frequencies, where the processing techniques provide a determination of a gas saturation radial profile.